Auto merge of #5388 - rhenium:topic/defer-requiring-rubygems-spec_fet…

…cher, r=segiddins

Defer requiring rubygems/spec_fetcher until it becomes necessary

Avoid conflict between two versions of openssl gem on 'bundle exec' if
a newer and non-default version of openssl gem is installed to the
system. rubygems/spec_fetcher loads openssl through resolv and
securerandom when running with Ruby <= 2.4.

This is not a proper fix for #5235 as other standard libraries that
Bundler currently loads will be gemified as well in Ruby >= 2.5,
however, this will fix openssl's case.

Reference: rubygems/bundler#5235
Fixes: ruby/openssl#103

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Reproduce:

1. Install Ruby 2.4.0 (comes with openssl 2.0.2)
2. Run `gem install openssl` to install openssl 2.0.3
3. Create a Gemfile and run `bundle install`
4. Run `bundle exec ruby -e'require "openssl"'